You can probably replace it with a MRF455 instead,it will require different mounting, and some capacitor changes (as per the HR2510 / Lincoln mod), but I expect it would work, and I doubt the final would ever fail again.

Isn't there a mod around somewhere for the HR2510/HTX100 that replaces the 477 to 455 mod with a 1446? The IRF520s are not a pin for pin replacement for the 477 and you wind up with them "cross-legged".